Backspotfacing is a machining operation within inaccessible areas or locations. For example, assume that the back side or blind side of an obverse surface needs to be machined, e.g., the blind side has a surface to be machined to provide a cylindrical recess for receiving a washer-like seal. Assume further that the blind side or work surface is disposed within a limited working environment or an enclosure and that the only access to the work surface is through a constricted opening in the enclosure.
As will be clear to those skilled in the art, conventional boring devices such as drilling bits, countersinking tools or the like, are inappropriate for backspotfacing. For example, the seals contemplated herein have a flat bottom surface necessitating a machined flat mating surface which cannot be obtained by using a conventional drilling bit or countersinking tool. More importantly, as will be obvious to the skilled artisan, it is impossible to drive conventional drilling bits and countersinking tools when the power source is on the same side as the obverse surface and the bit or countersinking tool is positioned on the blind side.
To solve the problems of backspotfacing, inverted cutters or backspotfacers have been devised which have cutting flutes reversed as compared to conventional drilling bits and counter-sinking tools. In a typical backspotfacing operation using a conventional inverted cutter, an operator is positioned on the same side as the obverse surface. A pilot rod, attached at one end to a conventional source of rotational power, is passed through a pilot hole drilled in the center of the work surface to be finished. The inverted cutter is releasably attached to the other end of the pilot rod with its cutting surface positioned on the work surface to be machined. During the machining operation, the inverted cutter is drawn towards the operator, i.e., into the work surface, thereby forming the cylindrical recess desired.
Unfortunately, when machining abrasive materials, such as, e.g., aluminum, composite materials, etc., conventional inverted cutters wear out quickly necessitating either the replacement or the resharpening of the cutter. The replacement or resharpening of the cutter is time consuming and costly. Furthermore, each time the inverted cutter is changed or resharpened, an operator must set up properly in order to ensure that the desired cylindrical recess is obtained. Finally, the range of the depth of cut of conventional inverted cutters is limited resulting in a problem with chip clearance, i.e., too deep a cut makes it difficult for the chips, resulting from machining, to flow away from the work surface--a necessary phenomema ensuring that the machining process can continue efficiently.
Comparable problems have been experienced using conventional drilling bits and countersinking tools. To increase the useful life of these cutters, disposable cutting inserts have been used. The inserts have eliminated the relatively expensive grinding operations necessitated by a wearing down of the original cutter. Examples of cutting tools having disposable flat chip-like cutting inserts include U.S. Pat. Nos. 4,355,932; 4,318,647; 4,293,252; 4,268,198; 4,224,029; 4,218,162; 4,210,406; 4,197,042; 4,194,862; 4,133,399; 4,124,328; 3,999,452; 3,963,365; 3,957,388; 3,400,616; 3,299,752 and 3,295,187. Unfortunately, these conventional cutters are inappropriate for backspotfacing, as explained above.
To date efforts at increasing the useful life of conventional inverted cutters have been limited to brazing carbide cutting edges to a cutter and to constructing expensive cutters made of special alloys. Conventional steels are generally inappropriate because of their useful life and inability to maintain a sharp edge. While the special alloy cutters stay sharp for much longer periods of time, the cost of purchase, frequency of replacement and resharpening are undesirably high. Whether the special alloy inverted cutters are resharpened or replaced, additional set up time is still required.